1. Field of the Invention
The invention is in the field of data acquisition, and more specifically in the field of detection systems.
2. Description of the Related Art
X-ray images may be recorded using a sensor array of charge coupled devices (CCDs). Charge coupled devices typically detect light by measuring collection of charge on a capacitor. Detectable photons increase the rate of charge collection and a measurement of the total charge collected within a period of time is indicative of a number of photons sensed. Typically, when charge collection is measured, the collected charge is allowed to escape (flushed) from the capacitor so that another charge collection and measurement cycle can begin. Charge collection and measurement cycles are sometimes referred to as data acquisition “frames.”
Charge coupled devices normally have an internal dark current that results in a minimum charge collection rate that is independent of the presence of detectable photons. If the collected charge is not flushed periodically this dark current will result in an offset and eventual saturation of the sensor. Charge coupled devices configured for detection of x-rays are, therefore, regularly flushed. Flushing preferably occurs between periods of charge collection intended for photon measurement and may be required even when the sensor array is not in a photon detection mode.
In a typical array of charge coupled devices, flushing is controlled by an internal clock configured to generate an electrical signal called an internal sync pulse. The charge coupled devices are flushed whenever an internal sync pulse occurs. The internal sync pulse is typically also available as an output of the sensor array. This output allows other devices such as data acquisition systems and external control electronics to coordinate their activities with the sensor array. For example, the internal sync pulse is commonly used to indicate a start of a data acquisition frame (that includes activation of an x-ray source, and a charge collection and measurement cycle). Coordination between sensor flushing and data acquisition is important because it would be undesirable for the charge coupled devices to be flushed during periods of charge collection for measurement of x-rays. Conflicts between detector flushing and charge collection for measurement are avoided by having the external control electronics monitor the internal sync pulse. In alternative configurations, flushing is controlled by an external signal from the external control electronics. In these configurations, the external signal is used to trigger generation of internal sync pulses at times determined by the external control electronics.
The internal sync pulse is optionally also used for coordination of communication between an array of charge couple devices and external control electronics. For example, in some cases, timing of data transfer or command signals between the external control electronics and the array of charge couple devices is tied to the occurrence of internal sync pulses. In these cases, each transfer of data or command signal is timed according to an internal sync pulse. As a consequence, a series of commands and replies between the array of charge couple devices and the external control electronics may require a series of internal sync pulses and the time required for the series of commands and replies may be dependent on the frequency of internal sync pulses.
When the array of charge couple devices uses an external signal from external control electronics for generation of internal sync pulses, the timing of data acquisition events are controlled by the external control electronics. This relationship between the external control electronics and the array of charge couple devices may be characterized as a “master-slave” relationship, the external control electronics being the master and the array of charge couple devices being the slave. In other configurations or data acquisition modes, the external control electronics may operate as a slave to the array of charge couple devices.
This master-slave relationship can be a disadvantage when a data acquisition mode of the array of charge couple devices is changed. For example, a user may wish to change from a “normal fluoroscopy” mode to a “full resolution” mode. This data acquisition mode change may require that the operation of the charge couple devices be altered and that configuration data be communicated between the array of charge couple devices and external control electronics. Because the series of communications required for a change in data acquisition mode require a series of internal sync pulses and because there may be some time required for the array of charge couple devices to react to the communications, a data acquisition mode change may require that the array of charge couple devices be unavailable for data acquisition over a period of time during which several internal sync pulses would be generated. During and after the data acquisition mode change there may be an additional undesirable delay before the external control electronics and array of charge couple devices have reestablished their master-slave relationship and are thus sufficiently synchronized for data acquisition. The total delay resulting from communications requirements, reaction time, and resynchronization is a disadvantage of the prior art in that this delay reduces the possibility of being able to perform rapid changes between data acquisition modes. Rapid changes in data acquisition modes are desirable when, for example, a physician wishes to use a first mode for real time observation and a second mode for high resolution examination of an item of interest.